This application claims benefit of Japanese Patent Application No. 2000-342686 filed on Nov. 9, 2000, the contents of which are incorporated by the reference.
The present invention relates to elastic surface wave filter devices used for optical communication systems and mobile radio communication systems and packages for accommodating elastic surface wave filters.
Elastic surface wave (hereinafter referred to as SAW) filters have such features as small size, high stability, high accuracy, high reliability and freedom from adjustment, and find extensive applications as functional devices for controlling high-frequency signals. As an example, SAW filters are used as frequency selectors, filter devices, resonators, delay elements and functional devices.
SAW filters control electric high-frequency signals via elastic surface waves propagated along a substrate surface. As the substrate used is usually made of such piezoelectric materials as liquid crystal LBO, lithium tantailate (LiTaO3) and lithium niobate (LiNbO3). Substrates made of piezoelectric material is hereinafter referred to as chip substrate. FIG. 12 shows an SAW filter having a plurality of interdigital transducers (IDTS) 11 to 14, which are formed on a chip substrate 10 by using a thin film forming technique or a lithographic technique.
The SAW filter has a pair of interdigital transducers or a plurality of pairs of interdigital transducers juxtaposed in the direction of progress of elastic surface wave. In the example shown in FIG. 12, two pairs of interdigital transducers are provided. In surface wave propagation path as a region with the interdigital transducers 11 to 14 provided therein, the SAW filter converts an elastic surface are to an electric high frequency signal or converts an electric high frequency signal to an elastic surface wave and selects a frequency dependent on the line width of the interdigital transducers 11 to 14.
The SAW is usually used in a state accommodated in a package. An SAW filter in the state accommodated in a package is hereinafter referred to as SAW filter device. The interdigital transducers 11 to 14 are connected by bonding wires or the like to input or output signal electrode terminals provided on the package accommodating the SAW filter. In the SAW filter shown in FIG. 12, the interdigital transducers 11 and 14, for instance, are connected as an input and an output signal electrode to an input and an output signal electrode terminal, respectively. Also, the interdigital transducers 12 and 13 are connected as grounding electrodes to grounding electrode terminals.
Since the piezoelectric material has pyroelectricity, when the SAW filter device experiences a sudden temperature change in such ambient circumstances as when it is reflow soldered to mount it in a printed circuit board or the like or when its thermal shock test is done, it is subject to polarization and generation of charge on the surface of the chip substrate 10. Such charge generated on the surface of the chip substrate 10 generates an electric field in the interdigital transducers. When the electric field generated in a portion subject to utmost charge concentration reaches the electric breakdown level, electric discharge is caused among the interdigital transducers and, in the extreme case, destruction of the interdigital transducers due to the electric discharge occurs.
To prevent the electric discharge among the interdigital transducers, a ceramic package 91, as shown in FIG. 13, is used to accommodate the SAW filter. The package has a metal film 92 formed by plating or deposition on its inner bottom surface. More specifically, the ceramic package 91 used for accommodating the SAW filter 100 is such that the bottom surface of the chip substrate of the SAW filter 100 is in contact with the metal film 92 when the SAW filter 100 is accommodated. Thus, the grounding electrodes of the SAW filter are electrically connected to the metal film 92. In this package, charge generated, if any, due to polarization can escape to the metal film 92, thus preventing the destruction of the interdigital transducers 11 to 14.
A plastic package may be used to reduce the cost of the SAW filter. However, it is difficult to form a metal film by plating or deposition on the plastic package. That is, it is difficult to provide, on the package, a means for permitting escape of charge generated due to polarization. It is therefore inevitable, as shown in FIG. 14, that the SAW filter be packaged without presence of any metal film between plastic package 7 and SAW filter 100. In order that the plastic package 7 can be used, it is important that the structure of the chip substrate of the SAW filter 100 is free from polarization or capable of permitting escape of charge generated due to polarization.
An object of the present invention, accordingly, is to provide an SAW filter device, which has a structure capable of preventing destruction of the interdigital transducers by charge generated on the chip substrate due to polarization in such case as when mounting the SAW filter on substrate or when testing the same.
Another object of the present invention is to provide a package for accommodating an SAW filter, which is a plastic package and, nevertheless, is capable of permitting escape of charge generated due to polarization.
An SAW filter device with a chip substrate of a piezoelectric material having a plurality of interdigital transducers and accommodated in a plastic package according to the present invention, wherein a common potential means for providing a common potential in the interdigital transducers, a charge neutralizing means for neutralizing charge generated on the chip substrate due to polarization, or a charge escape means for causing escape of charge generated on the chip substrate due to polarization, is provided as an electric discharge preventing means for preventing electric discharge among the plurality of interdigital transducers on the chip substrate.
With the electric discharge preventing means provided on the chip substrate, it is possible, without provision of any measure for preventing electric discharge on the package side, to permit neutralization or escape of charge generated due to polarization in such case as when mounting the SAW filter on substrate or when testing the SAW filter device. This permits accommodation of the SAW filter in the plastic package. That is, it is possible even in the case of using a plastic package to prevent destruction of the interdigital transducers due to charge generated by polarization.
A package for accommodating an SAW filter in the inside according to the present invention, which comprises a terminal member made of a metal extending out of the package and extending into the package such as to form an L-shaped portion. Thus, it is possible to accommodate the SAW filter in the package such that the bottom surface of the chip substrate is in contact with the metal plate portion. Charge generated due to polarization thus is not concentrated on the side of the SAW filter chip substrate surface but is neutralized. This means that such status as charge concentration on particular potions of the interdigital transducers can be avoided. Consequently, it is possible to prevent electric discharge among the interdigital transducers.
SAW filter device according to a preferable embodiment has a SAW filter having the electric discharge preventing means formed on the chip substrate and accommodated in the above package. Since the bottom surface of the chip substrate is in contact with the metal plate portion, charge generated due to polarization thus is not concentrated on the side of the SAW filter chip substrate surface but is neutralized.